In the prior art, a wheel hub is coupled to a knuckle spindle by a nut and supported to be rotatable by the knuckle spindle in a front axle portion of a vehicle. FIG. 5 is a cross-sectional view showing a coupling structure in which a wheel hub is coupled to a knuckle spindle by a nut.
Bearings 11 and 12, which are arranged in a wheel hub 10, support the wheel hub 10 to be rotatable on a knuckle spindle 14 of a knuckle 13. The knuckle spindle 14 includes a projection 14a that projects from the wheel hub 10. Male threads (not shown) are formed on an outer surface of the projection 14a. A nut 15 is fastened to the projection 14a with a predetermined tightening torque so that a proper pre-load is applied to the bearings 11 and 12.
A wheel 16 and a brake drum 17 are fixed to the wheel hub 10 by a plurality of wheel bolts 18 and wheel nuts 19. In regard to such a structure coupling the wheel hub 10 to the knuckle spindle 14, patent document 1 discloses a technique for further fixing a lock plate to the nut 15. FIG. 6 is an exploded perspective view showing in perspective the structure of each element of the nut rotation restriction structure described in patent document 1.
As shown in FIG. 6, in the nut rotation restriction structure described in patent document 1, the knuckle spindle 14 that extends in the axial direction A includes a key groove 14b, which has an opening in a distal end surface 14c of the knuckle spindle 14 and extends in the axial direction A. Three female threaded portions 20 are arranged in an end surface 15a of the nut 15 in the circumferential direction at 120° intervals. A disk-shaped lock plate 25 includes a central portion with an insertion receptacle 26 fitted to the knuckle spindle 14. An engagement piece 27, which is engaged with the key groove 14b of the knuckle spindle 14, extends toward the center from the inner rim of the lock plate 25. Further, the lock plate 25 includes eighteen bolt holes 28 arranged at 20° intervals in the circumferential direction so that three of the eighteen bolt holes 28 would be opposed to the female threaded portions 20 of the nut 15 when the engagement piece 27 is engaged with the key groove 14b. 
When the wheel hub 10 is loosely coupled to the knuckle spindle 14, the nut 15 is fastened to the knuckle spindle 14 with a predetermined tightening torque that applies a proper pre-load to the bearings 11 and 12. Then, the engagement piece 27 is engaged with the key groove 14b, and the knuckle spindle 14 is fitted in the insertion receptacle 26 of the lock plate 25. Further, bolts 29 are fastened through the bolt holes 28 to the female threaded portions 20 to fix the lock plate 25 to the nut 15.
In such a structure, the engagement of the key groove 14b of the knuckle spindle 14 with the engagement piece 27 of the lock plate 25 fixed to the nut 15 restricts rotation of the nut 15. In addition, the bolt holes 28 of the lock plate 25 are arranged at narrower intervals than the female threaded portions 20 of the nut 15. This facilitates the alignment of the female threaded portions 20 with the bolt holes 28.